Ultrasensitive electrochemical sensor for mercury (II) based on target-induced structure-switching DNA

A novel electrochemical sensor has been developed for sensitive and selective detection of mercury (II) based on target-induced structure-switching DNA. A 33-mer oligonucleotide 1 with five self-complementary base pairs separated by seven thymine–thymine mismatches was first immobilized on the electrode via self-assembly of the terminal thiol moiety and then hybridized with a ferrocene-tagged oligonucleotide 2, leading to a high redox current. In the presence of Hg2+, mercury-mediated base pairs (T–Hg2+–T) induced the folding of the oligonucleotide 1 into a hairpin structure, resulting in the release of the ferrocene-tagged oligonucleotide 2 from the electrode surface with a substantially decreased redox current. The response characteristics of the sensor were thoroughly investigated using cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The effect of the reaction temperature on the response of the sensor was also studied in detail. The results revealed that the sensor showed sensitive response to Hg2+ in a concentration range from 0.1 nM to 5 μM with a detection limit of 0.06 nM. In addition, this strategy afforded exquisite selectivity for Hg2+ against other environmentally related metal ions, which was superior to that of previous anodic stripping voltammetry (ASV)-based techniques. The excellent sensitivity and selectivity signified the potential of the sensor for Hg2+ detection in real environmental samples.